The present invention relates to the class of extruding apparatus commonly known as cross-head dies. One or more electrical wires or other elongated, filamentary members are moved axially through such apparatus as a flowable coating material is injected therein, and the coating material is applied in one or more substantially concentric layers to the filamentary member(s) as it exits the apparatus. This is the conventional manner of applying, for example, a plastic insulating layer to an electrical wire.
Cross-head die technology is normally used to produce an insulation layer surrounding an electrically conductive core filament. One inherent property of the process is the consistent ability to produce a uniform core-to-coating cross sectional area ratio. For this ability, the process has been recently adopted to manufacture cylindrical pellets containing long fiber glass cores surrounded by a thermoplastic matrix. Over 1000 lb./hr of composite material may be produced by simultaneously coating multiple glass strands using a multiple hole cross-head die. The invention revolutionizes the production of long fiber reinforced thermoplastic composites.
High quality long fiber composites require a consistent mass ratio of glass fiber to thermoplastic matrix. A mass ratio of 30 percent glass fiber to 70 percent thermoplastic resin is typical for the industry. Cross-head die technology, by maintaining stable cross-sectional areas and densities, produces an extruded, coated, fiber-reinforced matrix capable of segmentation into pellets. These pellets have consistent properties essential to the production of quality injection or compression molded products.
Prior art cross-head die apparatus typically comprises a plurality of elements cooperatively positioned within an axial bore of the body, and molten plastic is introduced through a radial bore. The elements are maintained in the desired relationship by mating, conically tapered surfaces.
Multiple filamentary members may be coated simultaneously by duplicating the axial bores in the body and the plurality of elements. The axial chambers communicate to a central flow chamber or manifold via a radial bore to receive a portion of the flowable resin.
Problems associated with prior art cross-head dies include, for example, low production rate, inequitable distribution of resin flow to each port, stagnation of flow within the flow chamber, difficulty in cleaning, and high initial cost.
Cross-head dies with single axial bores for the coating of elongated filamentary elements have limited volume production capability. The need for higher production rates led to the introduction of more complex, multiple cavity cross-head dies fed by a single flow chamber.
Flow chamber designs of multiple cavity cross-head dies vary greatly. Commonly known in the art "Fishtail" or "Headhanger" designs create stagnation points within the flow of resin. The stagnation points allow resin to collect and harden, producing clogged ports and costly downtime. The present invention eliminates stagnation points by providing a flow chamber with continuous linear flow. Each radial port is fed along a linear flow path with chamber termination at the final flow port. A single stagnation point may occur at the termination of the chamber. Chamber cleaning and removal of hardened material at this point is facilitated by removing a resin releasing screw located precisely at the potential stagnation. The linear flow chamber design reduces both the frequency of required cleaning and the difficulty in effecting such maintenance.
A limitation of the linear flow chamber is that resin head pressure drops along the flow path such that downstream radial ports receive less flow than upstream ports. This invention compensates for this limitation by providing variable restriction devices or adjustment screws at each port to balance the flow through each port. The adjustment screws allow individual control of resin flow to each port, whereby each filamentary member is coated evenly.
As a general rule, it is desirable to minimize the number of elements in a cross-head die. As with other structural assemblies, the parts reduction normally results in a reduction of initial cost, assembly and disassembly time, frequency of breakdowns and parts replacement. This assumes, of course, that the speed of operation, quality of final product, and the like are not compromised by elimination of certain elements. In particular, in operation of a cross-head die, it is essential to distribute the molten coating material evenly about the axially moving filamentary member.
In view of the foregoing, it is an object of the present invention to provide cross-head die assembly. It is another object of the present invention to provide a cross-head die assembly for applying a layer of coating material to a filamentary member moved axially through the die wherein the number of parts in the assembly is less than in prior art assemblies of the same class of apparatus. Another object is to provide a cross-head die of simplified design and generally lower cost than prior art apparatus of the same type. Yet another object of the present invention is to provide a cross-head die assembly capable of coating multiple filamentary members simultaneously. Still another object of the present invention is to provide a cross-head die assembly capable of high volume production. Still another object of the present invention is to provide a cross-head die assembly that requires a minimum of cleaning in operation. Yet another object of the present invention is to provide a cross-die assembly that may be easily and quickly repaired. A further object is to provide a cross-head die assembly with individual resin flow control to each radial port and axial bore.